For many years, resonant power converters have been increasingly used for drive engineering as an alternative to a conventional the traditional voltage-applying pulse-width-modulation inverters. Their purpose is to drastically reduce switching losses and optimally utilize the ensuing advantages--higher power density and higher operating frequency. Furthermore, semiconductors can switch higher currents because of the lower load during switching operations, thus permitting higher utilization of capacity.
Disadvantages include, in some cases, a considerable increased expense for active and passive components which must be added to shape the voltage and current characteristics appropriately during switching operations. In some cases higher peak currents and voltages occur on the switching elements. Additionally, completely different control methods than in the past must be used in some resonant power converters.
A power converter having only minor restrictions compared to other circuits is known as an auxiliary resonant commuted-pole (ARCP) power converter. In an ARCP power converter, the peak loads that occur on the switching elements are not higher than in a pulse-width-modulation inverter, and the conventional control methods can be used and need only be adapted with regard to dead times and minimum pulse periods. The increased expense in terms of components and control electronics is moderate and must be considered in relation to the advantages that can be achieved, optionally also in comparison with traditional balancing networks.
An article entitled "The auxiliary resonant commutated pole converter" by R. W. De Doncker et al., IEEE-IAS Conference Proceedings 1990, pp. 1228-35, describes an operation of an ARCP converter. In such an ARCP converter, a resonant capacitor is connected in parallel with each power semiconductor switch. Furthermore, an auxiliary circuit including an auxiliary switch which is connected in series with a resonant inductor is provided, connecting a neutral point of an indirect d.c. link capacitor to an output terminal of the converter phase. Two semiconductor switches with antiparallel diodes are provided as auxiliary switches; they are connected in series so that their cathodes, emitters and source terminals are linked together. The semiconductor switches used may be SCRs (symmetrically blocking thyristors), GTOs (gate turn-off thyristors), ZTOs or MCTs (MOS-controlled thyristors). The power semiconductor switches provided may be GTOs (gate turn-off thyristors), MCTs (MOS-controlled thyristors), IGBTs (insulated gate bipolar transistors) or PTRs (power transistors).
When power is to be supplied to high-power three-phase loads, e.g., three-phase machines in the MW range using self-commutated rectifiers, phase-to-phase voltage in the high-voltage range is demanded. To meet this demand at the current level of semiconductor technology, semiconductor components may have to be connected in series.
An article entitled "Series connection of IGBTs in resonant converters," by M. Dehmlow et al., IPEC Yokohama '95, pp. 1634-38, describes a resonant converter. Each of the resonant converter's, bridge branch valves includes a series connection of three IGBTs. Each power switch is provided with a resonant capacitor.
The expanded three-phase bridge connection is an advantageous option for a series connection of two components per bridge branch. With this circuit, the voltage of the a.c. terminals with respect to terminal 0 may assume the three values +U.sub.d /2, zero, and -U.sub.d /2. Therefore, this is also called a self-commutated three-point power converter or three-point power inverter.